I just got Ubuntu the other month or so. I'm running the 11.04 version currently and trying to install drivers for a Nvidia graphics card (it's my brother's computer and he currently cannot find the number of the card right now).

I read that, to install the proper drivers for gaming, I would need to go into "Additional Drivers" and choose them myself. I clicked to go into "Additional Drivers" and it searched "for available drivers", then opened the additional drivers list. Lo and Behold, there was nothing, simply the message "No proprietary drivers are in use on this system" and the Help and Close buttons.

Sorry if there's some silly simple answer to all of this, I'm new to this sort of thing (used to Windows simplicity, lol) and I really don't know what to do. Help would be much appreciated.

Also, I should mention that the graphics card was installed and ran fine before switching from Windows XP to Ubuntu.

Of course there is a driver in the kernel, but "Additional Drivers" exists for installing proprietary drivers that in most cases provide better performance. A huge part of modern proprietary graphics drivers are optimizations for certain programs. Sometimes those optimizations can be called cheating.
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LiveWireBTSep 21 '12 at 9:37

If nothing shows up in the additional drivers try
sudo apt-get install mesa-utils
this should show you the drivers if they are present in the kernel
Otherwise refer to this
Why aren't 3d effects working?

I found nothing in the "additional drivers" for my HP Pavilion 17 f1053us. I was getting frustrated by the 800x600 resolution. I managed to get Synaptic installed (Ubuntu Software Center is fundamentally inoperable at a low resolution). I used synaptic to install anything/everything that seemed relevant to ATI.

I then rebooted the machine and it came up in the native 1600x900 display mode! Perhaps this will work for you.